Field of the Invention
In the case of the application of lasers for treating or processing substrates, it is frequently necessary to apply lasers to the substrate in a plurality of target zones simultaneously or virtually simultaneously. The prior art discloses various refinements for this purpose in which in each case an incoming laser beam is swiveled with the aid of a rotating polygonal mirror over a row of juxtaposed, focusing elements, an appropriate number of target points being affected sequentially (see Published, French Patent Application FR 2 130 698 A, European Patent EP 0 028 615 B1, Published, Non-Prosecuted German Patent Application DE 195 11 393 A1). The known devices can be used to produce a plurality of tracks of perforated holes next to one another in a web-shaped material. The low level of energy that is required to produce a perforated hole permits the use of very high rotational speeds for the polygons. The hole densities within a perforation track are therefore very high, being up to 30 holes per centimeter of material, as is required, for example, in perforating cigarette tip paper.
However, substantially lower hole densities, that is to say much larger hole spacings, are required for perforating packaging materials such as, for example, plastic films, without aiming at a substantial change in the web speed and energy per hole. The lower hole densities serve the purpose of a higher tear resistance and a lower permeability.
It would certainly be possible in the case of the devices, known from the prior art, for treating substrates to achieve the desired large hole spacings in conjunction with low rotational speeds of the polygons, but the pulse energies would then be much too high while still achieving a clean hole. Counteracting this would require a reduction of the output power of the lasers, but this is not possible, or not possible to the required extent. It is not presently possible to manage an increase in the hole spacing by increasing the winding rate of the web-shaped material of the web to be perforated, because speeds of several thousand meters per minute would be required.
For specific applications, the prior art also discloses polygonal mirrors that are inclined in a specific way and with reference to the respective application. Published, Non-Prosecuted German Patent Application DE 40 09 113 A1 discloses a rotating polygonal mirror for a laser printer in the case of which the flat mirrors disposed around the axis of rotation and with their surfaces parallel to the axis of rotation are separated from one another by small connecting faces which have a surface, preferably a reflecting surface, which does not run parallel to the axis of rotation. As a result of this, the light reflected from the connecting faces does not impinge on the light-sensitive layer of the laser printer, but only the light scanned by the genuine reflecting faces. The aim is for the small connecting faces to reduce the air resistance resulting from the abutting edges between adjacent reflecting faces, and to reduce the noise produced during rotation. The inclination of the connecting faces results in that the light scanned on these faces cannot form ghost images on the light-sensitive bodies, and so the print quality of the laser printer is not worsened. Published, Non-Prosecuted German Patent Application DE 197 13 826 A1 describes a radar device which has a rotating polygonal mirror with a plurality of reflecting surfaces, of which each has a different inclination with reference to the axis of rotation of the polygonal mirror. A horizontal scanning operation is executed by rotating the polygonal mirror, while a vertical scanning operation is carried out by the reflecting surfaces with the various inclinations. At any time at which an incident beam is emitted toward a different reflecting surface, the horizontal scanning operation is repeated at a different height level that is proportional to the angles of inclination of each reflecting surface.
It is accordingly an object of the invention to provide a device for treating a substrate by laser radiation which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the perforation patterns with a larger hole spacing than previously possible can be produced without the need to vary substantially the web speed and energy per hole.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for treating a substrate. The device contains a laser beam source outputting a laser beam, a system of focusing positive lenses disposed next to one another and having focal planes lying on a surface of the substrate to be treated, and a rotating polygonal mirror with N reflecting faces disposed upstream of the system of focusing positive lenses. The rotating polygonal mirror swivels the laser beam coming from the laser beam source over the system of focusing positive lenses and the system of focusing positive lenses providing various output beam paths. The rotating polygonal mirror has a number of active reflecting faces from which the laser beam is directed onto the system of focusing positive lenses. The rotating polygonal mirror has inactive reflecting faces with a different inclination than the active reflecting faces.
The main advantage of the present invention resides in the fact that a specific, preferably small, number of active reflecting faces are provided, of which the laser beam is reflected in the direction of the substrate. That is to say the active face of the polygonal mirror which is used for treating substrates is of comparatively small dimensions. A further advantage of the invention resides in the fact that during the time when the inactive reflecting faces are being swept over, the possibility arises for switching on and off the laser. The switching behavior of all commercially available CO2 lasers does not permit the laser light to be switched on and off in the case of transition at the abutting edges of adjacent reflecting faces without the production of irregularities in the perforation. With the aid of these pauses, which become available owing to the inactive reflecting faces, a time domain is entered which is longer temporally by powers of ten and within which it is possible to switch CO2 lasers on and off without delays and transient phenomena becoming visible in the perforation track.
Depending on the application, it can also be provided that a plurality of working planes are defined, that is to say there is a plurality of rows of positive lenses disposed next to one another, and each row is assigned a specific, preferably equal, number of active reflecting faces, the active reflecting faces having a different inclination for each row.
In accordance with an added feature of the invention, an absorber is disposed downstream of the rotating polygonal mirror and onto which the laser beam reflected from the inactive reflecting faces is directed.
In accordance with an additional feature of the invention, the system of focusing positive lenses contains a plurality of rows of focusing positive lenses disposed next to one another, and in that each row is assigned a specific number of the active reflecting faces, the active reflecting faces having a different inclination for each row.
In accordance with another feature of the invention, at least one of the inactive reflecting faces is disposed between adjacent ones of the active reflecting faces.
In accordance with a further feature of the invention, N is an even number, and in that two mutually opposite active reflecting faces are provided.
In accordance with another added feature of the invention, N is an even number, and in that two mutually opposite inactive reflecting faces are provided.
In accordance with another additional feature of the invention, there is an optical measuring device for detecting positions of the active and the inactive reflecting faces.
In accordance with another further feature of the invention, the optical measuring device contains a light source for emitting a measuring beam onto the polygonal rotating mirror, and a photodetector for detecting a measuring beam reflected from the polygonal rotating mirror.
In accordance with a further added feature of the invention, the optical measuring device has a slotted diaphragm disposed in front of the photodetector, and a position of the slotted diaphragm or the photodetector can be varied parallel to an axis of rotation of the polygonal rotating mirror.
In accordance with a concomitant feature of the invention, there is an electronic control connected to the laser beam source for switching on and off the laser beam when it sweeps over the inactive reflecting faces.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for treating a substrate by laser radiation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.